I was comparing HAproxy to Squid a while ago and could not figure out what's haproxy's advantage over squid? I ended up using Squid but still am very interested in HAproxy, would like to learn more about it.

Squid remains to be the only one that can deal with SSL proxying(yes it's kind of MITM, but it's needed sometimes), and it's also the real "pure" open source. HAproxy might be better fit for enterprises that need support?

HAProxy is a TCP/HTTP load balancer, so the entire perspective of the software (from configuration to feature emphasis) stems from that.

Squid is a caching HTTP proxy, which began with forward proxying but also supports reverse proxying. I wouldn't regard it as relevant to modern, dynamic architectures as HAProxy or Varnish (another caching-focused project).

There's no real difference in open source purity between any of these projects, unless you dislike the stewardship of a company. HAProxy has existed for a long time without such stewardship (as has Varnish). Indeed, Squid's lack of commercial backing might be a hint as to its current relevance.

I disagree, Poul-Henning Kamp's HTTPS/HTTP2 rant is well known, but he's not going to abandon the project. He has steered it to follow the unix ideals of doing one thing well. They (varnish) forked stud and bought it up to date as hitch [1] which covers TLS. At some point they will incorporate HTTP2 once the demand is there.